The invention relates to an electro-optical liquid-crystal switching element comprising at least one polariser and a liquid-crystal layer which has an initial alignment in which the liquid-crystal molecules are aligned essentially parallel to the substrates and essentially parallel to one another, in which the realignment of the liquid crystals from their initial alignment essentially parallel to the substrates is caused by a corresponding electric field, which, in the case of liquid-crystal materials of negative dielectric anisotropy, is aligned essentially parallel to the substrates and, in the case of liquid-crystal materials of positive dielectric anisotropy, is aligned essentially perpendicular to the substrates, where the liquid-crystal layer has an extremely low optical retardation d-An in the range from 0.06 xcexcm to 0.43 xcexcm, and the liquid-crystal switching element preferably contains, in addition to the liquid-crystal layer, a further birefringent layer, preferably a xcex/4 layer or two xcex/4 layers or a xcex/2 layer, and 2 liquid-crystal display systems containing liquid-crystal switching elements of this type.
The present invention furthermore relates to liquid-crystal media, in particular of low birefringence, for use in the liquid-crystal display systems. These liquid-crystal display systems containing the liquid-crystal switching elements are, inter alia, display screens of television sets, computers, such as, for example, notebook computers or desktop computers, central control units and of other equipment, for example gambling machines, electro-optical displays, such as displays of watches, pocket calculators, electronic (pocket) games, portable data banks, such as PDAs (personal digital assistants) or of mobile telephones.
In particular, the liquid-crystal display systems according to the invention are highly suitable for applications with display of grey shades, such as, for example, television sets, computer monitors and multimedia equipment. Both mains-independent operation and also operation on the mains are possible here. Mains operation is often preferred.
These liquid-crystal display devices are also known as liquid-crystal displays.
The liquid-crystal switching elements typically used in the liquid-crystal display devices of this type are the known TN (twisted nematic) switching elements, for example in accordance with Schadt, M. and Helfrich, W. Appl. Phys. Lett. 18, pp. 127 ff (1974) and in particular in their special form with low optical retardation dxc2x7xcex94n in the range from 150 nm to 600 nm in accordance with DE 30 22 818, STN (super twisted nematic) switching elements, such as, for example, in accordance with GB 2.123.163, Waters, C. M., Brimmel, V, and Raynes, E. Pproc. 3rd Int. Display Research Conference, Kobe 1983, pp. 396 ff and Proc. SID 25/4, pp. 261 ff, 1984, Scheffer, T. J. and Nehring, J. Appl. Phys. Lett. 45, pp. 1021 ff, 1984 and J. Appl. Phys. 58, pp. 3022 ff, 1985, DE 34 31 871, DE 36 08 911 and EP 0 260 450, IPS (in-plane switching) switching elements, as described, for example, in DE 40 00 451 and EP 0 588 568, and VAN (vertically aligned nematic) switching elements, as described, for example, in Tanaka, Y. et al. Taniguchi, Y., Sasaki, T., Takeda, A., Koibe, Y., and Okamoto, K. SID 99 Digest pp. 206 ff (1999), Koma, N., Noritake, K., Kawabe, M., and Yoneda, K., International Display Workshop (IDW) ""97 pp. 789 ff (1997) and Kim, K. H., Lee, K., Park, S. B., Song, J. K., Kim, S., and Suk, J. H., Asia Display 98, pp. 383 ff, (1998).
In these liquid-crystal display devices which were known hitherto and are for the most part already commercially available, the optical appearance is inadequate, at least for demanding applications. In particular the contrast, especially in the case of coloured displays, the brightness, the colour saturation and the viewing-angle dependence of these parameters are in clear need of improvement and have to be improved if the display devices are to compete with the performance features of the widespread CRTs (cathode ray tubes). Further disadvantages of the liquid-crystal display devices are often their poor spatial resolution and inadequate response times, in particular in the case of STN switching elements, but also in the case of TN switching elements or IPS (xe2x80x9cin-plane switchingxe2x80x9d) and VAN (vertically aligned nematicxe2x80x9d) switching elements, in the case of the latter especially if they are to be used for the reproduction of video, such as, for example, in multimedia applications on computer display screens or in the case of television sets. Particularly for this purpose, but also even for the display of rapid cursor movements, short response times, preferably of less than 32 ms, particularly preferably of less than 16 ms, are desired.
The requirements regarding the viewing-angle dependence of the contrast are highly dependent on the application of the display devices. Thus, for example, the horizontal viewing-angle range is the most important in television screens and computer monitors, whereas centrosymmetrical or at least approximately centrosymmetrical viewing-angle distributions are desired in other applications. Displays having virtually centrosymmetrical viewing-angle distributions are required, in particular, in projection displays in order to utilise the optical apertures as well as possible, but also in computer display screens having a swivel base. These display screens allow the display to be tilted through 90xc2x0 in order to change from portrait mode to landscape mode while retaining the resolution of the display. Displays of this type obviously have to have similar horizontal and vertical viewing-angle ranges since these are interchanged on tilting.
In general, it should be noted that for practical acceptance of a display, it is not primarily its contrast or its maximum contrast ratio that is crucial, but instead the viewing-angle dependence of the contrast is frequently important. However, these properties should be weighted differently depending on the application.
TN switching elements having a dxc2x7xcex94n in the range from 0.2 xcexcm to 0.6 xcexcm, as described in DE 30 22 818, generally have very good colour saturation and colour depth, but an inadequate viewing angle for demanding applications, such as, for example, desktop computer monitors.
In some embodiments, such as, for example, in typical IPS display devices, the brightness of the display can be achieved to an inadequate extent or can only be achieved at great expense with backlighting. In contrast, VANs are frequently characterised by inadequate colour saturation and colour depth, and furthermore the production of VANs is complex owing to the homeotropic alignment, which is difficult to achieve, and owing to the long filling times.
EP 0 264 667 describes TN cells having twist angles (xcfx86, known as twist for short) in the range from 100 to 80xc2x0 with a dxc2x7xcex94n in the range from 0.2 xcexcm to 0.7 xcexcm. Although these have both an improved viewing-angle dependence of the contrast and lower steepness of the electro-optical characteristic line compared with TN cells having a 90xc2x0 twist, they have, however, significant disadvantages. Thus, inter alia, their brightness and their contrast are significantly lower than those of conventional TN switching elements. In addition, the TN switching elements in accordance with EP 0 264 667 switch relatively slowly.
Raynes, E. P., Mol. Cryst. Liq. Cryst. 4, p. 1, ff, 1986, describes the voltage dependence of the tilt angle in the centre of the liquid-crystal layer (xcfx86M, also known as mid-plane tilt angle or mid-plane tilt for short) as a function of the addressing voltage for cells containing a nematic liquid crystal with a tilted alignment having a tilt angle of from 0xc2x0 to 270xc2x0.
DE 40 10 503 and WO 92/17831, which corresponds thereto, describe, inter alia, TN switching elements having twist angles in the range from greater than 0xc2x0 to 90xc2x0 which contain one or more compensation layers, where the compensation layers for compensation of the optical path difference of the switching cell have the same optical retardation as the switching cell. In cells having a twist angle regarded as small, for example 22.5xc2x0, the compensation layer may also be omitted. However, the switch elements described in this publication have, in particular, inadequate contrast, which is frequently accompanied by a still considerable viewing-angle dependence of the contrast. Furthermore, the response times, in particular those for the addressing of grey shades, are usually inadequate.
DE 42 12 744 proposes improving the viewing-angle dependence of the contrast and in particular the display of grey shades by TN cells having a 90xc2x0 twist and a dxc2x7xcex94n in the range from 0.15 xcexcm to 0.70 xcexcm by using a cholesteric liquid-crystal material having a small cholesteric pitch (P) with a d/P ratio in the range from 0.1 to 0.5. The TN switching elements of DE 42 12 744 exhibit similar disadvantages to the switching elements described in EP 0 264 667. The saturation voltage also increases significantly in the cells in accordance with DE 42 12 744 compared with conventional TN cells, albeit not so highly pronounced as in the TN switching elements of EP 0 264 667.
WO 91/06889 and the corresponding U.S. Pat. No. 5,319,478 describe [lacuna] the minimum optical retardations of xcex/2 or xcex/4 and propose their operation with circular-polarised light. Cells having a twisted structure of the liquid crystal are preferred.
Van Haaren et al., Phys. Rev. E, Vol. 53, No. 2, pp. 1701 to 1713, investigates the elastic constants for surface coupling (k13) of the nematic liquid-crystal mixture ZLI-4792, Merck KGaA, in an untwisted cell with a planar alignment having a xcex/4 plate.
Tillin et al., SID 98 Digest, pp. 311-314 (1998), investigates reflective liquid-crystal switching elements having a single polariser. He mentions, inter alia, a liquid-crystal switching element having an untwisted liquid crystal which switches from a (dxc2x7xcex94n/xcex) of xc2xd to xc2xc in normally white mode and from xc2xc to 0 in normally black mode. However, the publication prefers liquid-crystal layers having a twisted structure. In addition, liquid-crystal cells having an (dxc2x7xcex94n/xcex) of ⅓ containing a birefringent layer having a (dxc2x7xcex94n/xcex) of xc2xd and optionally an additional birefringent layer having a (dxc2x7xcex94n/xcex) of {fraction (4/55)} are presented. In these, the characteristic directions of the optical components form angles to one another which differ by 0xc2x0 and 90xc2x0. The switching elements having birefringent layers which are described here have a complex structure and are consequently not easy to produce. In addition, the brightness is not particularly good, in particular in the switching elements having a plurality of birefringent layers.
It has now been found that the liquid-crystal switching elements according to the present invention do not have the disadvantages of the known switching elements or at least do so to a significantly reduced extent. They are characterised by very good contrast at the same time as excellent viewing-angle dependence of the contrast. They allow the display both of grey shades and of half-tone colours over a broad range of observation angles. In addition, the response times are good and in particular are adequate for video reproduction.
The liquid-crystal switching elements according to the present invention contain a liquid-crystal layer having a small optical retardation and, if desired, a further birefringent layer, preferably a xcex/4 layer, a xcex/2 layer or two xcex/4 layers, and at least one polariser. The two xcex/4 layers may replace the xcex/2 layer.
The transmissive or transflective liquid-crystal switching elements according to the present invention preferably contain a polariser and an analyser, which are arranged on opposite sides of the arrangement of liquid-crystal layer and birefringent layer. Polariser and analyser are jointly referred to as polarisers in this application.
FIG. 1 shows a diagrammatic view of the principle of construction of a liquid-crystal switching element according to the invention in the preferred embodiment of a transmissive switching element having a light source, a having a liquid-crystal layer, having two polarisers, having a birefringent layer (here, as preferred, a xcex/4 layer) and having crossed polarisers.
FIG. 1a is a side view. For reasons of clarity, the substrates of the liquid-crystal cell between which the liquid-crystal layer is located and the electrode layers located on the alignment layers present on the insides of the substrate and on one or both substrates are omitted. Each of the two polarisers is located on one of the two sides of the liquid-crystal cell. The birefringent layer is located between the liquid-crystal cell and one of the two polarisers, preferably, as shown, on the side facing away from the light source, i.e. between the liquid-crystal cell and the analyser. In this configuration, the fast axis of the birefringent layer is parallel to the transmission axis of the polariser. The light from the light source (backlighting, BL for short) thus passes successively through the polariser, the liquid-crystal cell, the birefringent layer and the analyser before reaching the observer (not shown). However, it is also possible to reverse the sequence of liquid-crystal layer and birefringent layer. In this case, however, the relative alignment of these two components also has to be changed. The fast axis of the birefringent layer is then preferably at an angle of 45xc2x0 to the polariser, and the projection of the alignment of the liquid crystal in the centre of the cell between the substrates is preferably parallel to the transmission direction of the polariser.
FIG. 1b shows a plan view, i.e. along the z-axis in FIG. 1a. It shows the alignment of the relevant axes of the individual optical components to one another and defines the corresponding angles. The symbols from FIG. 1a are used where appropriate. xcexa8PP denotes the angle between the transmission axes of the two polarisers (here 90xc2x0), xcexa8PL denotes the angle between the transmission axis of the polariser and the preferential direction of the liquid-crystal director in the centre of the layer between the substrates (n∥) (here 45xc2x0). The fast axis of the xcex/4 layer is parallel to the transmission axis of the polariser. The angle xcexa8PD is thus 0xc2x0. Finally, the observation angle in the plane of the switching element ("PHgr") is indicated with examples of 0xc2x0, 90xc2x0, 180xc2x0 and 270xc2x0.
The observation angles in the plane of the display ("PHgr" and "PHgr"xe2x80x2) and perpendicular to the normal ("THgr") are defined in FIG. 2. The observation angles "PHgr"xe2x80x2 commence with "PHgr"xe2x80x2=0xc2x0 in the quadrant with the highest contrast at the angle of the highest contrast, which is generally in the direction of n∥. Thus, [lacuna] or "PHgr"xe2x80x2 is shifted by 45xc2x0 compared with "PHgr".
The fast axis of the xcex/4 layer is parallel to the transmission direction of the polariser, to that of the polariser adjacent to the xcex/14 layer in the case of the presence of two or more polarisers (cf. FIG. 1b). An analogous situation applies in the presence of two xcex/4 layers or of one xcex/2 layer.
Linear polarisers are preferably used in the switching elements according to the present application. These linear polarisers may be single-layered polarisers or consist of a combination of a plurality of layers, where these layers may also comprise two or more polarising layers. The degree of polarisation of the polarisers is chosen to be sufficiently high in order to achieve good contrast, but also sufficiently low to achieve good brightness of the switching element. The use of a polariser having a relatively low degree of polarisation, a so-called clean-up polariser, in combination with a polariser having a relatively high degree of polarisation often proves advantageous. In this case, the polarisers are preferably bonded using an adhesive of appropriate refractive index in order to avoid light losses at the surfaces.
The liquid-crystal layer is usually held between two substrates. At least one of the substrates transmits light, preferably both substrates transmit light. The light-transmitting substrates consist, for example, of glass, quartz glass, quartz or of transparent plastics, preferably of glass and particularly preferably of borosilicate glass.
The substrates together with an adhesive frame form a cell in which the liquid-crystal material of the liquid-crystal layer is held. The substrates are preferably planar.
The separation of the planar substrates is kept essentially constant over the entire area by means of spacers. These spacers may be used only in the adhesive frame or, alternatively, distributed over the entire area of the cell. The use of spacers exclusively in the adhesive frame reduces problems with misalignment in the liquid-crystal layer. It is particularly appropriate in the case of liquid-crystal cells having small area diagonals, in particular up to 5xe2x80x3 and preferably up to 3xe2x80x3. In the case of larger-area liquid-crystal cells, in particular in the case of those having diagonals of 14xe2x80x3 or more and very particularly of 18xe2x80x3 or more, spacers are preferably employed distributed over the entire area. In this case, it is possible and often advantageous to employ different spacers in the adhesive frame and in the cell area. The preferred limits for the various distributions of the spacers over the cell area additionally depend on the thickness of the substrates used. Thus, the use of spacers distributed over the entire display area is preferred in the case of relatively thin glass and in the case of relatively large diagonals.
The preferred substrate thicknesses are from 0.3 mm to 1.1 mm, particularly preferably from 0.4 mm to 0.7 mm. In the case of the relatively large diagonals of the cells, the substrates of relatively large thicknesses are preferably employed.
The liquid-crystal switching elements according to the invention are distinguished by very good grey-shade capacity, a low dependence of the contrast on the observation angle, even in the case of colour display, with a large viewing-angle range and low contrast inversion and, in particular, by very short response times. In particular, the inverse contrast, as defined, for example, in DE 42 12 744, which occurs, for example, in displays in accordance with DE 30 22 818, is significantly reduced, in particular at relatively large observation angles xcex8.
As spacers, commercially available spacers in bead form or in cylindrical form may consist either of plastics or of inorganic materials, such as, for example, chopped glass fibre. Suitable spacers are furthermore more or less regular, raised structures on, preferably, one of the substrates. These regular, raised structures may have various shapes, such as, for example, rectangular, square, oval or round columns or pyramid shafts, but also strip- or wave-shaped structures.
The liquid-crystal switching elements in accordance with the present application have, if they are reflective switching elements, at least one polariser and a reflector, with at least one polariser and the reflector being located on opposite sides (i.e. substrates) to one another in the liquid-crystal cell. In the case that they are transmissive or reflective switching elements, these have at least two polarisers, in each case at least one of which is arranged on one of the two opposite sides of the liquid-crystal cell (so-called sandwich structure). The obligatory polarisers mentioned are preferably linear polarisers and particularly linear polarisers having a high degree of polarisation.
In addition to the obligatory polarisers, the switching elements according to the invention may contain one or more further polarisers. These may be so-called clean-up polarisers having a lower degree of polarisation, but high transmission. In particular in the case of reflective switching elements, however, a further polariser having a high degree of polarisation may also be present. This is preferably arranged between the liquid-crystal cell and the reflector. However, the use of additional polarisers is generally less preferred since in most cases it results in a reduction in the transmission. However, it is usual, in particular in connection with so-called brightness-increasing components, which may, for example, contain cholesteric polymer films.
In the case of transmissive and transflective displays in accordance with the present application, the two obligatory polarisers are arranged either crossed or parallel to one another. In this application, the directions of the arrangement of the polarisers are relative to their absorption axes. The crossed arrangement of the polarisers is preferred. The angle of the absorption axes to one another (xcexa8PP) in the case of crossed polarisers is in the range from 75xc2x0 to 105xc2x0, preferably from 85xc2x0 to 95xc2x0, particularly preferably from 88xc2x0 to 92xc2x0, especially preferably from 89xc2x0 to 91xc2x0 and very particularly preferably 90xc2x0, and in the case of parallel polarisers is from xe2x88x9215xc2x0 to 15xc2x0, preferably from xe2x88x925xc2x0 to 5xc2x0, particularly preferably from xe2x88x922xc2x0 to 2xc2x0, especially preferably from xe2x88x921xc2x0 to 1xc2x0 and very particularly preferably 0xc2x0.
The angle between the absorption axis of the polariser adjacent to the liquid-crystal layer with the direction of the alignment of the director of the liquid-crystal material in the unswitched (field-free) state at the adjacent substrate (xcexa8PL) is from 35xc2x0 to 55xc2x0, preferably from 40xc2x0 to 50xc2x0, particularly preferably from 43xc2x0 to 47xc2x0, in particular from 44xc2x0 to 46xc2x0 and ideally 45xc2x0. This applies to the untwisted alignment of the liquid crystal. In the case of the twisted alignment of the liquid crystal, the preferential direction for the indication of the angle xcexa8PL is the projection of the alignment of the liquid-crystal director in the centre between the two substrates of the cell on the substrate adjacent to the polariser. On use of further birefringent layers and/or compensators in addition to the obligatory or preferred xcex/4 or xcex/2 layer or layers, depending on the embodiment, other angles between the polariser direction and the liquid-crystal alignment can also be employed. However, these are generally not preferred.
The twist angle (xcfx86) of the liquid-crystal layer between the two substrates is, in particular in the case of switching elements having a birefringent layer, in particular having a xcex/4 or xcex/2 layer, or having a plurality of birefringent layers, in particular having two xcex/4 layers, preferably from xe2x88x9220xc2x0 to 20xc2x0, particularly preferably from xe2x88x9210xc2x0 to 10xc2x0, especially preferably from xe2x88x925xc2x0 to 5xc2x0, very particularly preferably from xe2x88x922xc2x0 to 2xc2x0 and most preferably from xe2x88x921xc2x0 to 1xc2x0.
For the preferred embodiment without a birefringent layer, i.e. without a xcex/4 or xcex/2 layer or layers, the liquid-crystal layer is essentially untwisted and particularly preferably untwisted. A twist angle (xcfx86) of from xe2x88x926xc2x0 to 6xc2x0 is preferred. The twist angle is particularly preferably from xe2x88x921.0xc2x0 to 1.0xc2x0, very particularly preferably from xe2x88x920.5 to 0.5, especially preferably 0.0xc2x0.
The liquid-crystal materials are aligned at the substrate surfaces by conventional methods. To this end, use can be made of inclined vapour deposition with inorganic compounds, preferably oxides, such as SiOx, alignment on surfaces that have been subjected to antiparallel rubbing, in particular on polymer layers, such as polyimide layers, that have been subjected to antiparallel rubbing, or alignment on photopolymerised anisotropic polymers. In the case of vertical alignment (abbreviated to VA), it is also possible to employ lecithin or surface-active substances for homeotropic alignment.
The liquid-crystal switching elements in accordance with the present invention can be produced using the production processes in the production plants for the most widespread liquid-crystal switching elements to date, the TN liquid-crystal switching elements. In particular, there is no need for special effort regarding alignment of the liquid-crystal director, as, for example, in STN (high tilt angle) or in VAN (homeotropic alignment). In addition, in contrast to TN, IPS having a twisted initial state and in particular to STN, additives, such as chiral dopants, can be substantially and frequently even completely omitted. A further process parameter which is sometimes difficult to control is thus superfluous.
The surface tilt angle at the substrates (xcfx860, tilt for short) is in the range from 0xc2x0 to""15xc2x0, preferably in the range from 0xc2x0 to 10xc2x0, particularly preferably in the range from 0.1xc2x0 to 5xc2x0 and especially preferably in the range from 0.2xc2x0 to 5xc2x0 and most preferably in the range from 0.3xc2x0 to 3xc2x0. The surface tilt angle at the alignment layer on at least one of the substrate surfaces is from 0.5xc2x0 to 3xc2x0. The tilt angle at the two substrates is preferably essentially identical.
The electrodes on the substrates transmit light, at least on one of the substrates and preferably on both substrates. The material employed for the electrodes is preferably indium tin oxide (ITO), but it is also possible to use aluminium, copper, silver, and/or gold.
Since the surface tilt angle in the liquid-crystal display elements according to the invention may be small, the use of anisotropically photopolymerisable materials, such as, for example, cinnamic acid derivatives, the so-called photoalignment should particularly advantageously be employed.
This applies in particular to a preferred embodiment of the liquid-crystal display elements according to the invention, the embodiment with multidomain switching elements. In these, the individual liquid-crystal switching elements or their individual display electrodes (also known as pixels) are divided into sub-areas of different alignment of the liquid-crystal director, at least in the switched state, but generally also in the unswitched state, so-called domains. These domains having different alignment in the switched state can be induced, for example, by different surface tilt angles or by different differential alignment on the substrates. However, they can also be induced by corresponding electric fields with a sufficiently inclined alignment, for example through slotted electrodes, or through non-planar surface topographies. In particular in the case of induction of the domains by electric fields which are not perpendicular to the substrates, but also in the case of non-planar surface topographies, the smallest possible surface tilt angle, if possible of 0xc2x0, is usually preferred, as can readily be achieved by means of photoalignment. The individual pixels of the multidomain switching elements preferably contain two or more, preferably precisely a multiple of two, very particularly preferably two or four, domains. The tilt angles of the liquid-crystal director in the centre of the liquid-crystal layer (xcfx86M, mid-plane tilt angle) of these domains in the switched state are preferably opposite one another in pairs. The result of this is that the viewing-angle dependencies of the domains, also known as sub-pixels, cancel each other out, and the undesired effect is eliminated. The light-scattering disclinations which occur at the domain limits are covered by a corresponding mask, preferably a black mask, in order to improve the contrast. Through appropriate design of the structure or structures inducing the domains, and of the mask, the restriction in the light yield by the reduced aperture ratio can be kept as low as possible.
The larger of the preferred surface tilt angles are particularly advantageous for a definition of the preferred quadrant, i.e. the quadrant in which the best contrast is observed. They result, in particular, in a suppression of reverse tilt domains, which arise particularly easily on application of nonorthogonal fields.
Active electric switching elements of the active matrix which are used are both bipolar structures, such as diodes, for example MIM diodes or back to back diodes, if desired with reset, and tripolar structures, such as transistors, for example thin-film transistors (TFTs), or varistors. For the liquid-crystal display devices in accordance with the present application, TFTs are preferred. The active semiconductor medium of these TFTs is amorphous silicon (a-Si), polycrystalline silicon (poly-Si) or cadmium selenide (CdSe), preferably a-Si or poly-Si. Poly-Si here equally denotes high-temperature and low-temperature poly-Si.
In liquid-crystal switching elements according to a preferred embodiment of the present invention, the liquid-crystal layer preferably has an optical retardation (dxc2x7xcex94n) of from 0.14 xcexcm to 0.42 xcexcm, particularly preferably from 0.22 xcexcm to 0.34 xcexcm, especially preferably from 0.25 xcexcm to 0.31 xcexcm, very particularly preferably from 0.27 xcexcm to 0.29 xcexcm and ideally 0.28 xcexcm.
To this end, liquid-crystal materials of low birefringence xcex94n are preferably employed. The birefringence of the liquid-crystal materials is preferably from 0.02 to 0.09, particularly preferably from 0.04 to 0.08, especially preferably from 0.05 to 0.075, very particularly preferably from 0.055 to 0.070 and ideally from about 0.060 to 0.065.
The layer thickness of the liquid-crystal layer is preferably from 1 xcexcm to 10 xcexcm, preferably from 2 xcexcm to 7 xcexcm, particularly preferably from 3 xcexcm to 6 xcexcm and especially preferably from 4 xcexcm to 5 xcexcm.
In liquid-crystal display devices containing liquid-crystal cells having a diagonal of up to 6xe2x80x3, layer thicknesses of the liquid-crystal layer of from 1 xcexcm to 4 xcexcm and particularly from 2 xcexcm to 3 xcexcm are preferred. In liquid-crystal display devices containing liquid-crystal cells having a diagonal from 10xe2x80x3, layer thicknesses of the liquid-crystal layer of from 3 xcexcm to 6 xcexcm and particularly from 4 xcexcm to 5 xcexcm are preferred.
There are two different preferred sub-forms for this preferred embodiment. In the first of these preferred sub-embodiments of the present invention, the liquid-crystal layer has an optical retardation (dxc2x7xcex94n) of from 0.20 xcexcm to 0.37 xcexcm, preferably from 0.25 xcexcm to 0.32 xcexcm, particularly preferably from 0.26 xcexcm to 0.30 xcexcm, very particularly preferably from 0.27 xcexcm to 0.29 xcexcm, and most preferably 0.28 xcexcm.
In this preferred sub-embodiment, the display element surprisingly does not require a xcex/4 layer in some applications. It is nevertheless characterised by good brightness, excellent contrast and excellent viewing-angle dependence and very good grey-shade and colour-shade display given an appropriate polariser setting, preferably at an angle of essentially 45xc2x0 to the liquid-crystal preferential direction. Without a xcex/4 layer, a very broad viewing-angle range for the observation angle "THgr" is achieved, although not for all observation angles "PHgr". By contrast, the viewing-angle range in the switching elements having a xcex/4 layer is significantly more centrosymmetrical, i.e. extends to all similar, large values of the observation angle "THgr" at all observation angles "PHgr" (cf. in this respect also FIGS. 9a) and 9b) regarding Examples 1 and 2).
In the second of these preferred sub-embodiments of the present invention, the display elements preferably contain a xcex/4 layer, and the liquid-crystal layer has an optical retardation [(dxc2x7xcex94n)LC] of from 0.10 xcexcm to 0.45 xcexcm, preferably from 0.20 xcexcm to 0.37 xcexcm, particularly preferably from 0.25 xcexcm to 0.32 xcexcm, very particularly preferably from 0.26 xcexcm to 0.30 xcexcm, especially particularly preferably from 0.27 xcexcm to 0.29 xcexcm, and most preferably 0.28 xcexcm. The liquid-crystal layer in the unswitched state thus behaves approximately like a xcex/2 layer. Preference is furthermore given here to an embodiment in which the (dxc2x7xcex94n)LC is different from 0.28 xcexcm, preferably in the range from 0.10 xcexcm to 0.27 xcexcm or from 0.30 xcexcm to 0.45 xcexcm, particularly preferably from 0.14 xcexcm to 0.25 xcexcm or from 0.32 xcexcm to 0.42 xcexcm, very particularly preferably from 0.22 xcexcm to 0.25 xcexcm, or from 0.32 xcexcm to 0.34 xcexcm.
In the present application, the wavelength x always preferably relates to the wavelength of maximum sensitivity of the human eye, to 554 nm, unless explicitly stated otherwise.
The terms xcex/4 layer and xcex/4 plate, or xcex/2 layer and xcex/2 plate are generally used with equal importance in the present application. The term xcex in xcex/4 layer and xcex/2 layer denotes a wavelength in the region of xcexxc2x130%, preferably xcexxc2x120%, particularly preferably xcexxc2x110%, especially preferably xcexxc2x15% and very particularly preferably xcexxc2x12%. The wavelength here, unless stated otherwise, is 554 nm. The wavelength of the xcex/4 layer or xcex/2 layer is generally and in particular in the case of a significant spectral distribution given as the central wavelength thereof.
The xcex/4 layer or xcex/2 layer is an inorganic layer or preferably an organic layer, for example comprising a birefringent polymer, for example stretched films (PET) or liquid-crystalline polymers.
The use particularly of the smaller of the preferred layer thicknesses of the liquid-crystal layer is preferred in view of the advantageous short response times which can be achieved thereby. In addition, it tends to allow the use of conventional liquid-crystal materials or makes at least lower demands regarding the often difficult implementation of the small an values.
By contrast, the use of liquid-crystal materials having a particularly small xcex94n is preferred in view of the lower layer thickness dependence of the contrast and of the background hue of the liquid-crystal switching elements. In addition, the production of the display elements in this embodiment is possible with significantly greater yields, especially in the case of liquid-crystal cells having larger diagonals.
For a broad working-temperature range, particular preference is given to liquid-crystal materials having a relatively high clearing point, since the effect of the xcex/4 layer is significantly temperature-dependent, owing to the temperature dependence of the birefringence of the liquid-crystal materials [xcex94nLC(T)], and xcex94nLC(T) in liquid-crystal materials having a high clearing point is relatively low. The temperature dependence of the optical arrangement as a whole is thus kept relatively low and can thus, if necessary, also be compensated more readily.
In a second preferred embodiment of the present invention, the liquid-crystal layer has an optical retardation of from 0.07 xcexcm to 0.21 xcexcm, preferably from 0.11 xcexcm to 0.17 xcexcm, particularly preferably from 0.12 xcexcm to 0.16 xcexcm, especially preferably from 0.13 xcexcm to 0.15 xcexcm and very particularly preferably 0.14 xcexcm. In this preferred embodiment, the display element preferably has at least one birefringent layer, preferably a xcex/2 layer or two xcex/4 layers, in addition to the liquid-crystal layer.
To this end, liquid-crystal materials of low birefringence xcex94n are again preferably employed. The birefringence of the liquid-crystal materials is preferably from 0.02 to 0.09, particularly preferably from 0.04 to 0.08, especially preferably from 0.05 to 0.07, very particularly preferably from 0.055 to 0.065 and ideally about 0.060.
The layer thickness of the liquid-crystal layer is preferably from 0.5 xcexcm to 7 xcexcm, preferably from 1 xcexcm to 5 xcexcm, particularly preferably from 1.5 xcexcm to 4 xcexcm and especially preferably from 2 xcexcm to 2.5 xcexcm. Particular preference is given here to displays containing liquid-crystal cells having smaller diagonals, in particular in the range from 0.5xe2x80x3 to 6xe2x80x3, particularly in the range from 1xe2x80x3to4xe2x80x3.
In this second preferred embodiment, the liquid-crystal switching elements preferably contain two xcex/4 layers or, particularly preferably, one xcex/2 layer. The two xcex/4 layers can be used on different sides of the liquid-crystal layer, but they can also be located on the same side of the liquid-crystal layer.
In particular if the optical retardation of the liquid-crystal layer [(dxc2x7xcex94n)LC] is significantly different from 0.14 xcexcm, particularly if it is in the range from 0.07 xcexcm to 0.12 xcexcm or from 0.16 xcexcm to 0.21 xcexcm, the use of two xcex/4 layers, or one xcex/2 layer is necessary.
This second preferred embodiment makes high demands both regarding the birefringence of the liquid-crystal material and regarding the layer thickness of the liquid-crystal layer. However, the requirements of the layer thickness of the liquid-crystal layer are somewhat reduced by the lower layer-thickness dependence of the optical properties of the switching elements. In the case of small-area liquid-crystal cells, the layer-thickness tolerance is in addition easier to comply with. In addition, the thin liquid-crystal cells in this preferred embodiment have extremely short short response times.
The liquid-crystal switching elements in accordance with the present application can be operated transmissively, transflectively or reflectively. The transmissive or transflective mode, particularly preferably the transmissive mode, is preferred.
Transflective displays enable the advantages of low power consumption of the reflective displays to be combined with that of good legibility at low ambient brightness of the transmissive displays with backlighting.
Reflectors which can be used are dielectric or metallic layers. Metallic reflector layers are preferred. On use of metallic reflectors, a greater by, variation in the optical retardation of the liquid-crystal layer can be tolerated. If a dielectric mirror is used, the optical retardation of the liquid-crystal layer is essentially xcex/4, in particular in the case of switching elements without a birefringent layer. On use of a second linear polariser between the liquid-crystal layer and the reflector, preference is given to a dielectric reflector, which preferably has a low fraction of depolarised reflection.
Particularly preferred combinations of the optical retardation of the liquid-crystal layer and of the birefringent layer are shown in the following table (Table 1). In this table, the preferred settings of the polarisers both with respect to one another and with respect to the preferential direction of the liquid crystals are also indicated.
The angle xcexa8PD is preferably 0xc2x0+/xe2x88x925xc2x0, particularly preferably 0xc2x0+/xe2x88x922xc2x0 and very particularly preferably 0xc2x0+/xe2x88x921xc2x0.
The following table (Table 2) shows preferred combinations of the optical retardations of the liquid-crystal layer and, if present, of the birefringent layer with the twist angles of the liquid-crystal layer.
The liquid-crystal switching elements in accordance with the present invention act as light valves on application of a voltage. This is shown, for example, in FIGS. 1 and 2 for the liquid-crystal switching elements of the first preferred embodiment of the present application. With crossed polarisers, the switching element in the voltage-free state, the xe2x80x9coff statexe2x80x9d, transmits light (known as xe2x80x9cnormally whitexe2x80x9d or alternatively positive contrast). With increasing applied voltage, a threshold is initially reached from which the transmission begins to drop. The transmission then drops in a virtually linear manner with increasing voltage over a relatively broad voltage range. At higher voltage, the transmission comes up against a limit, and saturation is reached.
The liquid-crystal switching elements are preferably addressed in such a way that the optical retardation of the liquid-crystal layer in the case of complete switching approaches 0 nm or at least essentially 0 nm. This naturally does not exclude the addressing of grey shades with the intermediate values required for this purpose.
It goes without saying that in order further to improve the optical properties, the display elements in accordance with the present invention may contain further optical layers. These layers can be, for example, compensation layers, which are employed, in particular, in display elements having a twist of the liquid-crystal layer which is different from 0xc2x0, or alternatively films which collimate the light, for example from backlighting, such as the so-called xe2x80x9cbrightness enhancement filmsxe2x80x9d (BEF) or cholesteric circular polarisers for utilisation of the half of the backlighting light which is otherwise absorbed by the polariser.
The display of coloured images using the display elements in accordance with the present invention is possible in various ways. Backlighting having an approximately white spectral distribution is preferably used, and the colour splitting carried out by a colour filter. The individual liquid-crystal switching elements are then employed as light valves for the respective primary colours. The backlighting can also be matched to the spectral characteristics of the colour filter in such a way that it has corresponding intensity maxima in the respective transmission regions. However, colour display can also be achieved by birefringence effects.
The liquid-crystal switching elements according to the invention and in particular the reflective switching elements preferably operate in normally white mode (for the polariser setting, cf. FIG. 3 and the associated description).
Liquid-crystal mixtures which are used in the liquid-crystal switching elements according to the invention preferably comprise from 3 to 27, particularly preferably from 10 to 21 and very particularly preferably from 12 to 18, individual compounds. The individual compounds preferably employed preferably each contain a 1.4xe2x80x2-trans-trans-bicyclohexylene unit of the sub-formula i: 
where
Z is a single bond, xe2x80x94CH2CH2xe2x80x94 or xe2x80x94CF2CF2 and
n is 1 or 2.
It is also possible here for one or preferably two non-adjacent xe2x80x94CH2xe2x80x94 groups in one of the cyclohexane rings to be replaced by oxygen atoms or for two adjacent xe2x80x94CH2xe2x80x94 groups to be replaced by one xe2x80x94CHxe2x95x90CHxe2x80x94 group.
In the case of compounds having a total of only two six-membered rings, it is also possible, if desired, for one of the two cyclohexane rings to be replaced by 1.4-phenylene, which may also, if desired, be laterally difluorinated or preferably monofluorinated.
The liquid-crystal mixtures preferably comprise one or more compounds containing a structural unit of the formula i in which n is 2.
The liquid-crystal mixtures used in the liquid-crystal switching elements according to the invention preferably comprise
a component A consisting of compounds having 2 six-membered rings,
a component B consisting of compounds having 3 six-membered rings, and, if desired,
a component C consisting of compounds having 4 six-membered rings.
The liquid-crystal mixtures preferably essentially consist of components A, B and, if desired, C.
Particularly preferred liquid-crystal mixtures comprise one or more
dielectrically neutral compounds of the formula I 
in which
R11 is n-alkyl having from 1 to 5 carbon atoms,
R12 is n-alkyl having from 1 to 5 carbon atoms, 1E-alkenyl, preferably vinyl, or n-alkoxy having from 1 to 6 carbon atoms,
optionally dielectrically positive compounds selected from the group consisting of the formulae II and IIxe2x80x2 
in which
R21 is n-alkyl or 1E-alkenyl having from 3 to 7 or from 2 to 8, preferably from 5 to 7 or from 4 to 6 carbon atoms respectively,
Z2 is a single bond or xe2x80x94CH2CH2xe2x80x94
and
X2 is OCF3, CF3 or CH2CH2CF3, preferably CF3 or CH2CH2CF3, 
in which
R2xe2x80x2 is n-alkyl or 1E-alkenyl having from 3 to 7 or from 2 to 8, preferably having from 5 to 7 or from 4 to 6 carbon atoms respectively,
Z2xe2x80x2 is a single bond or xe2x80x94CH2CH2xe2x80x94,
X2xe2x80x2 is OCF2H, OCF3 or F, preferably F,
and
Y2xe2x80x2 and Z2xe2x80x2, independently of one another, are H or F,
and
compounds of the formula III 
in which
R31 is n-alkyl or 1E-alkenyl having from 2 to 7, preferably from 2 to 5, carbon atoms,
Z31 and Z32 may each be a single bond of Z31 and Z32 and xe2x80x94CH2CH2xe2x80x94 or xe2x80x94CF2CF2xe2x80x94, preferably xe2x80x94CH2CH2, but particularly preferably are both a single bond,
X3 is OCF2, OCF3 or F,
Y3 and Z3, independently of one another, are H or F,
in the case of
X3=OCF2 both Y3 and Z3 are preferably F,
in the case of
X3=F both Y3 and Z3 are preferably F,
in the case of
X3=OCF3 one of Y3 and Z3 is preferably F, and the other is H,
optionally one or more compounds selected from the group consisting of the compounds of the formulae IV and V 
in which
R4 is n-alkyl or 1E-alkenyl having from 2 to 5, preferably having from 2 to 5 carbon atoms, 
X4 is OCF2H, OCF3 or F, preferably F or OCF3,
Y4 and Z4, independently of one another, are H or F,
in the case of
X=F and 
both Y4 and Z4 are preferably F
in the case of
X=OCF3 and particularly preferably in the case of 
one of Y3 and Z3 is F, and the other is H. 
in which
R5 is n-alkyl or 1E-alkenyl having from 2 to 5 carbon atoms,
Z51 is a single bond or xe2x80x94CH2CH2xe2x80x94,
X5 is F, OCF3 or OCF2H,
Y5 and Z5, independently of one another, are H or F,
preferably
X5, Y5 and Z5 are all F,
optionally one or more compounds of high clearing point selected from the group consisting of the compounds of the formulae VI to XI 
in which R71 and R72, R81 and R82, R91 and R92, R10 and R11 are each, independently of one another, as defined above for R11 and R12 in the formulae I,
L81 and L91 are H or F, and
X10, Y10 and Z10, and X11, Y11 and Z11 are each, independently of one another, as defined above for X3, Y3, and Z3 in the formulae III.
The liquid-crystal mixtures in accordance with the present application preferably comprise from 4 to 36 compounds, particularly preferably from 6 to 25 compounds and very particularly preferably from 7 to 20 compounds.
Particularly preferred liquid-crystal mixtures comprise one or more compounds selected from the group consisting of the following compounds from Table 3 and especially preferably in each case one or more compounds of at least three, preferably of at least four, different formulae of those listed in Table 3 below.
The temperature range of the nematic phase preferably extends from xe2x88x9220xc2x0 C. to 60xc2x0 C., particularly preferably from xe2x88x9230xc2x0 C. to 70xc2x0 C. and very particularly from 40xc2x0 C. to 80xc2x0 C. The birefringence is preferably from 0.040 to 0.070, particularly preferably from 0.050 to 0.065 and very particularly preferably from 0.054 to 0.063. The rotational viscosity is preferably from 60 to 170 m Pa s, particularly preferably from 80 to 150 m Pa s and very particularly preferably from 90 to 139 m Pa s. The threshold voltage (V10) in the switching elements according to the invention is preferably from 0.9 V to 2.7 V, particularly preferably from 1.1 V to 2.5 V and very particularly preferably from 1.2 V to 2.0 V. The sum response times for switching between V10 and V90 and back in the switching elements according to the invention are preferably at most 100 ms, particularly preferably at most 80 ms, very particularly preferably 60 ms or less. For faster applications, the sum response times are 50 ms or less, preferably 45 ms or less, particularly preferably 40 ms or less, especially 40 ms or less and very particularly preferably 30 ms or less.
Furthermore, the preferred parameters of the liquid-crystal mixtures can readily be gathered by the person skilled in the art from the examples shown below. In particular, the preferred ranges for the physical properties of the liquid-crystal mixtures and their combinations are those which are covered by the values in the examples.
The liquid-crystal mixtures particularly preferably essentially consist of compounds selected from the group consisting of the compounds of the formulae I, II, IIxe2x80x2 and III to XI.
The liquid-crystal media employed in the liquid-crystal switching elements in accordance with the present invention preferably consist of from 3 to 35 compounds, particularly preferably of from 4 to 25, very particularly preferably of from 5 to 20 and especially preferably of from 6 to 15 compounds.
The preferred d/P range is from xe2x88x920.25 to 0.25. For the lowest possible addressing voltages, a d/P in the range from xe2x88x920.1 to 0.1, particularly 0, is preferred. For optimum display of grey shades and in order to suppress inverse contrast, d/P values having a figure of from 0.1 to 0.25, particularly from 0.15 to 0.24, are preferred.
In the present application, the following apply, unless expressly stated otherwise:
the physical properties were determined as described in: Merck Liquid Crystals, Physical Properties of Liquid Crystals, Description of the Measurement Methods, Ed. W. Becker, Status Nov. 1997,
all physical data are given for a temperature of 20xc2x0 C.,
all temperatures are given in xc2x0 C. and all temperature differences in differential degrees,
all concentration data are in % by weight,
xcex94n (xcex94n=n∥xe2x88x92nxe2x8axa5) relates to 589.3 nm,
xcex94xcex5 (xcex94xcex5=xcex5∥xe2x88x92xcex5xe2x8axa5) relates to 1 kHz,
xcex31: rotational viscosity,
kil: elastic constants,
xcex is 576 nm,
V0: capacitive threshold or alternatively Freedericks threshold,
V10: threshold voltage (for 10% relative contrast, "THgr"=0xc2x0),
V50: mid-grey voltage (for 50% relative contrast, "THgr"=0xc2x0),
V90: saturation voltage (for 90% relative contrast, "THgr"=0xc2x0),
xcfx84delay: dead time from 0% to 10% change in the relative contrast,
xcfx84rise: rise time from 10% to 90% change in the relative contrast,
xcfx84on: switch-on time from 0% to 90% change in the relative contrast,
xcfx84off: switch-off time from 90% to 10% change in the relative contrast,
xcfx84sum: sum response time=xcfx84on+xcfx84off,
"PHgr"and "PHgr"xe2x80x2: observation angle in the display plane,
"THgr": observation angle from the display normal,
xcfx86: twist angle of the liquid-crystal director between the two substrates,
xcfx86: tilt angle of the liquid-crystal director,
xcfx860: tilt angle of the liquid-crystal director at the substrate surface or at the alignment layer,
xcfx86m: tilt angle of the liquid-crystal director in the centre of the liquid-crystal layer,
xcexa8PP (identical with xcexa8PA): angle between the transmission axes of the polarisers,
xcexa8PD: angle between the transmission axis of the polariser and the fast axis of the birefringent layer,
xcexa8PL and xcexa8AL: angle between the transmission axis of the polariser or of the analyser and the alignment direction of the liquid-crystal material at the respective adjacent substrate,
the electro-optical properties and response times were determined with rectangular alternating voltage addressing with a frequency of 60 Hz,
the stated voltage values are root mean square (rms) values,
xe2x80x9cessentially 0xe2x80x9d means, unless stated otherwise, 0+/xe2x88x921, preferably 0+/xe2x88x920.1 and particularly preferably 0+/xe2x88x920.1,
xe2x80x9cessentiallyxe2x80x9d in connection with physical properties means, unless stated otherwise, with a deviation of not greater than +/xe2x88x9210%, preferably +/xe2x88x925% and particularly preferably +/xe2x88x922% of the respective value,
xe2x80x9cessentially consisting ofxe2x80x9d means, unless stated otherwise, that the proportion of other constituents is not greater than 10%, preferably not greater than 5% and particularly preferably not greater than 2%,
the numerical values given in the present application are, unless stated otherwise, accurate to +/xe2x88x92 one unit in the final place given,
limits of the ranges indicated are, unless stated otherwise, inclusive, but preferably exclusive,
 greater than = and  less than =,  greater than /= and  less than /=, and xe2x89xa7 and xe2x89xa6 in each case mean less than or equal to or greater than or equal to respectively, and
+/xe2x88x92 means plus or minus.
The rotational viscosity of the nematic liquid-crystal mixture ZLI-4792 (Merck KGaA) at 20xc2x0 C. using the calibrated rotational viscometer was 133 mPaxc2x7s.
The electro-optical properties were investigated in test cells from Merck KGaA""s production:
Layer thickness:
Glass: borosilicate glass with a thickness of 1.1 mm (Pilkington)
ITO: 100 ohm/square inch (i/square inch)
Alignment layer: AL-1054 from Japan Synthetic Rubber, Japan,
Tilt angle: from 1xc2x0 to 2xc2x0 (determined using liquid-crystal material ZLI-4792 from Merck KGaA, Germany,
Twist angle: 0xc2x0 (glass plates subjected to antiparallel rubbing),
d/P: 0 (undoped).
The optical and electro-optical properties of the test cells were measured in commercial instruments from Autronic-Melchers, Karlsruhe, Germany (DMS 301 and DMS 703) and in addition in a home-made instrument from Merck KGaA, in each case using white light. The home-made instrument uses a photomultiplier as detector and a filter for matching the addressing sensitivity of the detector to the sensitivity curve of the human eye.
In the home-made instrument from Merck KGaA, the xcex/4 layer was permanently mounted as a platelet in the ray path. In the case of the measurements with the DMS 703, a xcex/4 film of liquid-crystalline polymer from Merck Ltd, Great Britain, was used.
In the present application and in particular in the examples, the liquid-crystal compounds are denoted by abbreviations. The coding of the structures is obvious and is carried out in accordance with Tables A and B. All groups CnH2n+1, CmH2m+1 ClH2l+1 and CkH2k+1 are straight-chain alkyl chains having n, m, l and k carbon atoms respectively. The coding in Table B is self-explanatory. Table A shows only the respective skeletons of the structures. The individual compounds are indicated below through specification of the designation of the core followed, separated by a dash, by the designation for the substituents R1, R2, L1 and L2:
The liquid-crystal mixtures of the present invention preferably comprise:
four or more compounds selected from the group consisting of the compounds from Tables A and B and/or
five or more compounds selected from the group consisting of the compounds from Table B and/or
two or more compounds selected from the group consisting of the compounds from Table A.
The effect of the present invention is illustrated below with reference to figures and examples and compared with the prior art.